Wireless cellular communication networks incorporate large numbers of mobile user equipment (UEs) and a number of base nodes (NodeBs). A NodeB is generally a fixed station, and may also be called a base transceiver system (BTS), an access point (AP), a base station (BS), or some other equivalent terminology. As improvements of networks are made, the NodeB functionality evolves, so a NodeB is sometimes also referred to as an evolved NodeB (eNB). In general, NodeB hardware, when deployed, is fixed and stationary, while the UE hardware is typically portable.
In contrast to NodeB, the mobile UE can comprise portable hardware. User equipment (UE), also commonly referred to as a terminal or a mobile station, may be fixed or mobile device and may be a wireless device, a cellular phone, a personal digital assistant (PDA), a wireless modem card, and so on. Uplink communication (UL) refers to a communication from the mobile UE to the NodeB, whereas downlink (DL) refers to communication from the NodeB to the mobile UE. Each NodeB contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the mobiles, which move freely around it. Similarly, each mobile UE contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the NodeB. In cellular networks, the mobiles cannot communicate directly with each other but have to communicate with the NodeB.
With each successive cellular phone handset generation, users demand more features in a smaller form factor. Some recent examples include cell phones with integrated Bluetooth, GPS, digital camera, and MP3 functionality. Process shrinks help deliver a cost and size advantage for digital designs with relative ease. However, for analog/RF designs, the immaturity of advanced processes comes with design challenges that may outweigh the intended advantage. In a typical handset, 30 to 40% of handset board space is occupied by analog/RF functionality which cannot be re-designed or migrated to the newer process/technology nodes easily, inhibiting vendor ability to cost effectively add features and reduce footprint.
Digital radio has recently allowed the replacement of space consuming analog RF circuitry with much more compact digital circuitry, thereby facilitating the ability to port designs rapidly to more advanced lithographies. Texas Instruments (TI) has proven this concept with its Digital RF Processor (DRP™) architecture, which it has successfully implemented in production versions of its Bluetooth BRF6xxx transceivers, GSM/GPRS LoCosto TCS23xx transceivers among other chips. DRP implementation is consistent with the on-going trend toward RF-CMOS in the cellular area, making it attractive in terms of power consumption, cost, and the integration of multiple radios.
Transmitters use one or more amplifiers, such as a digital pre-power amplifier (PPA) and an external power amplifier (PA), to amplify components of the input signal to be transmitted. These components are in-phase and quadrature components in the case of a Cartesian transmitter.
A highly linear amplifier distorts the signal the least and so is most favored from a standpoint of signal quality. Unfortunately, highly linear amplifiers use relatively large amounts of power and numbers of highly accurate and tightly matched components, making them relatively power consumptive, large and expensive. Though they perform the best, they are thus disfavored in many wireless applications, particularly those that require low-cost transmitters or transmitters that are subject to large operating voltage excursions. The amplifier that is best suited overall for low-cost, battery-powered wireless transmitters is a simpler amplifier having significant nonlinearities.
Predistortion is often used to compensate for these nonlinearities, resulting in a linearization of the output of the amplifier. The theory underlying predistortion is that if an amplifier's distortion characteristics are known in advance, an inverse function can be applied to an input signal to predistort it before it is provided to the amplifier. Though the amplifier then distorts the signal as it amplifies it, the predistortion and the amplifier distortion essentially cancel one another, resulting in an amplified output signal having substantially reduced distortion.
In digital transmitters, digital predistortion (DPD) is most often carried out using a lookup table (LUT) that associates output values with input signal values. Entries in the LUT are addressed using samples of the input signal. The output values retrieved from the LUT are used either to modify the samples (an “inverse gain” configuration) or in lieu of the samples (a “direct mapping” configuration). In modern applications such as WCDMA, samples are transmitted at a very high rate. Thus, the predistorter needs to be able to look up and retrieve output values very quickly.
WCDMA Cartesian transmitters suffer nonlinearities resulting from both amplitude modulation (AM) and phase modulation (PM), namely AM-AM and AM-PM interactions, occurring in their amplifier(s). In such Cartesian transmitters, predistortion is carried out at least partially to negate the effect of these interactions.
Values for a nominal predistortion LUT are typically computed during initial factory calibration. Unfortunately, a factory-calibrated predistortion LUT often fails to linearize the amplifier(s) adequately under varying operational conditions (e.g., temperature, voltage, frequency and voltage standing-wave ratio, or VSWR). Aging, especially in WCDMA and other so-called “3G” transmitters, only exacerbates the inadequacy.